The image on a television screen consists of picture elements (pixels), arranged horizontally in rows, generally offset vertically by one pixel position from one another. Each pixel is assigned a value which indicates, for a color image, the intensity of the red, green, or blue components of the pixel. A video image is generated by sequentially displaying the rows of pixels as horizontal lines of the image.
Existing analog broadcast standards such as NTSC, PAL and SECAM use two interlaced video fields to generate a single video frame. Each field includes one-half of the horizontal lines that make up the image frame. One field includes all of the odd numbered lines of the frame and the other field includes all of the even numbered lines. Interlaced images may exhibit distorting artifacts such as line flicker or loss of vertical detail that degrade the resulting frame image. One way to eliminate these artifacts is to convert the interlace-scanned fields into progressive-scan frames. In a progressive-scan frame, both the odd and even image lines are displayed concurrently as a single image.
Interlace-scan to progressive-scan conversion systems are gaining importance as more television viewers purchase high-definition television monitors that can display progressive-scan signals. Both broadcast facilities and consumers may want to have interlace-scan to progressive-scan conversion capability in order to avoid the distorting artifacts of interlace-scan images.
The simplest method of generating the interpolated pixel is simply duplicating the pixel from the corresponding position in the previously received scan line. For pixels which lie on an edge, this could result in “jaggies” (a line which appears to be jagged or stair-stepped, rather than smooth). For pixels which are not on an edge, such duplication could result in pixels that do not correspond to the image being displayed, resulting in a poor display to the viewer. This method also reduces the vertical resolution of the image compared to an interlace-scan image and may result in areas of the image flickering at a 30 Hz rate.
Another way to generate a progressive-scan frame from an interlace-scan field is to interpolate interstitial lines in each field. Thus, the lines of the odd field are used to interpolate even-numbered lines and the lines of the even field are used to interpolate odd-numbered lines. Each pixel of the interpolated line (i.e. each “interpolated pixel”) is calculated based on the values of proximate pixels in adjacent interlace-scan lines. While this method tends to reduce the visibility of the jaggies, it also reduces the vertical resolution of the image, resulting in an undesirable blurring.
One problem with using interpolation occurs when the interpolated pixel is on the edge of two visually distinct regions. Interpolating such a pixel may result in a pixel that matches neither adjacent pixel. For example, the value generated for an interpolated pixel between a blue pixel and green pixel would be cyan, which would not result in the image desired to be presented to the viewer.
Another problem may occur when images of moving objects are displayed. If the interpolation method does not adequately compensate for the change in the presented image, then an interpolated pixel may appear to be uncoordinated with its surrounding pixels. For example, when a black dog in successive frames moves from left to right in front of a white wall, the left portion of the image which had displayed a black pixel must now display a white pixel, or the sense of movement may not be adequately conveyed to the viewer.
It would be desirable to implement this conversion with a minimum of processing time and overhead, and retain a high degree of accuracy in developing the desired visual image.